Wireless body area sensor network (“WiBaSe-Net”) [1] is an emerging technology that can be used in medical, entertainment, and fitness applications. In a WiBaSe-Net, several wearable or implanted sensor devices, for instance electrocardiogram (“ECG”) sensor, blood pressure sensor, blood glucose sensor, temperature sensor, respiratory sensor, pulse oximeter, and accelerometer, are deployed throughout the body. A body controller unit (“BCU”) collects data from the sensor devices and sends it to the medical center.
The IEEE 802.15.1 [2] and the IEEE 802.15.4 [3] based technologies will be suitable for WiBaSe-Nets [4]. IEEE 802.15.4 supports not only contention based access mechanism but also guaranteed time slot (“GTS”) scheme under beacon-enabled mode for delay-sensitive applications.
GTS transmission can avoid packet drop due to collisions in the contention-based protocol (i.e., “CSMA/CA”), limited number of allowable retransmissions and number of back-offs as specified in the standard. In a medical sensor network, GTS allocation can also reduce the energy consumption of the sensor nodes due to carrier sensing. The IEEE 802.15.4 standard allocates GTS slots to devices in first-come first-serve (“FCFS”) fashion. The IEEE 802.15.4 standard has been developed for use in wireless personal area networks (“WPAN”). This standard only supports a limited number of GTS for time-critical or delay-sensitive data transmission. Due to the lack of optimization, this standard allocation results in wastage of bandwidth while serving asymmetric traffic from different sensor devices.
IEEE 802.15.4[3] specifies the MAC protocol for low data rate short-range wireless networks such as medical wireless sensor networks. It can support two operating modes: beacon-enabled and non-beacon-enabled modes. In non-beacon-enabled mode, the unslotted carrier can sense multiple access with collision avoidance (“CSMA/CA”) protocol is used. In beacon-enabled mode, the network coordinator can transmit a beacon to synchronize and provide necessary information to the devices.
As shown in FIG. 1, time can be divided into superframes. A superframe can be divided into active and inactive periods to maintain the low duty cycle. The active period can be divided into 16 equal slots. The active period can consist of two parts: a contention access period (“CAP”) and a contention free period (“CFP”). During CAP, the wireless devices can use a slotted CSMA/CA protocol. All the control commands and request commands can be transmitted during CAP. CFP can be used if a device is assigned with a dedicated GTS. The coordinator can allocate a maximum of seven GTSs to the devices. The CFP starts immediately after CAP and finishes at the end of the active period. During CFP, transmissions can be performed without using CSMA. GTS can be allocated for either uplink or downlink transmission. The coordinator can maintain the beacon interval (“BI”), the slot length, the active period (“SD”), and the inactive period in the beacon order (“BO”) and superframe order (“SO”) as shown in FIG. 1.
Whenever a device requires a certain guaranteed bandwidth for transmission, the device can send a GTS request command using CSMA/CA during CAP. Upon receiving the request, the coordinator first checks the availability of GTS slots in which the length of CAP must not be shorter than aMinCAPLength. The coordinator informs the device about the allocation of slot in the GTS descriptor in the next beacon frame (FIG. 1). If the device is not included in GTS descriptor for aGTSDesPersistenceTime superframes, then the GTS request is considered to be failed. GTS de-allocation can be performed by the coordinator or by the device itself. For device initiated de-allocation, it sends GTS request with characteristic type subfield set to zero using CSMA/CA during CAP. Similarly, if the coordinator does not receive data from the device in the GTS for at least 2 f superframes, the coordinator will de-allocate the GTS with starting slot subfield set to zero in the GTS descriptor field of the beacon frame for that device where f=28·BO for 0≦BO≦8, and f=1 for 9≦B≦14.
As pointed out in [6], since the slot length is fixed to a specific duty cycle, GTS slots may be under-utilized due to low data rate transmission and packet with the size smaller than slot length. In addition, regardless of data rate requirement and traffic congestion, the coordinator assigns GTS to the requested devices on the first-come first-serve basis. Therefore, a high data rate device may not receive the GTS allocation.
The problem of GTS allocation was addressed in the literature. A GTS allocation and priority updating scheme was presented in [7] taking latency and fairness of data transmission into account. In [8], i-GAME scheme was proposed to improve the GTS utilization. In this scheme, GTS is shared among multiple devices in a round-robin fashion. This scheme allows more than seven devices to use GTS simultaneously. In [9] an algorithm for GTS allocation during CFP was proposed for the IEEE 802.15.4 standard. This allocation is based on the payload, number of requested slots, and the delay constraint for data transmissions. A method to improve bandwidth utilization of GTS was presented in [10] by dividing the CFP into 16 slots for simultaneous transmissions without any change in GTS descriptor format. In [6] the GTS characteristic field was restructured to accommodate the information about payload demand, delay constraint, and number of periods which can be used to improve bandwidth utilization. However, a device can have only limited choices of payload demand, delay and number of periods due to limited number of bits available in the GTS characteristic field.
GTS allocation schemes were proposed in [8] and [9] considering the delay-guaranteed service. In these schemes, the information of delay requirements needs to be exchanged with the controller which incurs signalling overhead. The scheme in [9] also has high computational complexity due to the execution of a number of algorithms. The scheme in [8] requires each requesting node to identify flow specification which incurs additional control overhead.
It is, therefore, desirable to provide a method and system for allocating guaranteed time slots for time-critical or delay-sensitive data transmission in IEEE 802.15.4 wireless personal area networks to improve the reliability and bandwidth utilization in these networks.